1. Field of the Invention
The invention relates in general to an ultra-thin sensing device with a flat contact surface, and more particularly to an ultra-thin sensing device, which has a flat contact surface and has a sensing chip using vertical through electrodes.
2. Related Art
Recently, mobile devices, such as mobile phones, tablet computers or smart watches, are increasingly developed, and the thickness of the mobile device is getting thinner and thinner. Correspondingly, the thickness of the integrated circuit (IC) package used in the mobile device is requested to be thin. In addition, users have paid more and more attention to the outlooks of the mobile devices. So, if a sensor, such as a fingerprint sensor, is to be mounted on the mobile device, the sensor must not damage the outlook of the mobile device. Consequently, the sensor must have an all-flat (fully flat) design and a considerably thin thickness, so that the sensor can be hidden in, for example, a front or rear cover or a button of the mobile device.
In a conventional capacitive/electric field fingerprint sensor, sensing circuits and sensing members are formed on a silicon substrate, on which output/input bonding pads are also formed, wherein the bonding pads on the silicon substrate are electrically connected to package bonding pads on the package substrate by way of wire bonding, and then the encapsulating process to produce a fingerprint sensor device. However, under the restriction of the existing wire-bonding technology, the wire inevitably has an arc section curved upwardly from the bonding pad, and this arc section is disadvantageous to the all-flat sensor, and the sum of the height of the arc and the height of the encapsulating layer is greater than 100 microns. This is also disadvantageous to the ultra-thin design. In addition, the encapsulating layer with the height of 100 microns also significantly decreases the sensing capacitance and thus affects the sensitivity. Thus, it is improper for the sensing device to be hidden in the front/rear cover or button of the mobile device.
In another fingerprint sensor, solder balls of a ball grid array (BGA) are used to bond the silicon substrate of the chip to the package substrate. However, the diameter of the solder ball generally ranges from 300 to 400 microns (um). The total thickness of the fingerprint sensor is substantially equal to the sum of the diameter of the solder ball, the thickness of the chip and the thickness of the package substrate, and is inevitably greater than 700 um or even 1000 um. This is disadvantageous to the requirement of the thinned product.